1. Field of the Invention
The present invention relates to an optical system, and more particularly, to an optical system suitable for an image pickup optical system of a silver-halide film camera, a digital still camera, or a video camera or for a projection optical system of a projector, and an optical apparatus including the optical system.
2. Description of the Related Art
In recent years, with the improvement of image quality of a digital image pickup apparatus, an image pickup element has been reduced in size. In particular, a size (area) per pixel has been reduced. With this reduction, an image pickup lens is also required to be a high-performance lens in which various aberrations are further reduced as compared with conventional lenses.
Up to now, a retrofocus (negative-lead-type) lens is known as a lens system advantageous to achieve a wide angle (wide field angle). In the retrofocus lens, a lens unit having a negative refractive power in total is provided on the front side in the optical system (on the subject side in a case of an image pickup optical system of a camera or the like, or on the screen side in a case of a projection optical system of a projector or the like).
A lens unit having a positive refractive power is provided on the rear side in the optical system (on the image side in a case of an image pickup optical system of a camera or the like, or on the original image side in a case of a projection optical system of a projector or the like), to thereby realize a short focal length and a long back focus.
The retrofocus lens has an asymmetrical refractive power arrangement in which the lens unit having the negative refractive power is provided on the front side in the optical system, and hence a negative distortion (barrel type distortion) is likely to be generated.
In order to reduce the negative distortion, a negative lens of the lens unit having the negative refractive power may be desirably made of a high-refractive index material. However, the high-refractive index material generally has high dispersion characteristic, and hence a negative lateral chromatic aberration is likely to be generated.
Japanese Patent Application Laid-Open No. H09-033801 discloses a retrofocus lens for use in the image pickup optical system. The retrofocus lens has a single focal length and includes a first lens unit having a negative refractive power and a second lens unit having a positive refractive power, which are provided in the stated order from the object side to the image side.
In Japanese Patent Application Laid-Open No. H09-033801, in order to correct a lateral chromatic aberration, the first lens unit having the negative refractive power is provided and a positive lens made of high-dispersion glass is provided in a lens unit close to the first lens unit. However, in such a structure, a high-order lateral chromatic aberration still remains without being corrected sufficiently enough. Therefore, a lateral chromatic aberration of a g-line in a high-image height portion (large-field angle region) is varied in a positive direction, and hence an excessive correction may be made.
Japanese Patent Application Laid-Open No. H06-082689 discloses a retrofocus lens for use in the projection optical system. The retrofocus lens has a single focal length and includes a first lens unit having a negative refractive power and a second lens unit having a positive refractive power, which are provided in the stated order from the screen side to the original image side.
In Japanese Patent Application Laid-Open No. H06-082689, in order to correct a negative lateral chromatic aberration, a positive lens made of a low-dispersion material with extraordinary partial dispersion is provided in the rear of an aperture stop in which a height “h” of a paraxial chief ray entering the lens (distance from an optical axis) is relatively high. However, in such a structure, a high-order lateral chromatic aberration still remains without being corrected sufficiently enough. Therefore, a lateral chromatic aberration of a g-line in a high-image height portion (large-field angle region) is varied in a positive direction, and hence an excessive correction may be made.
U.S. Pat. No. 7,426,079 discloses a retrofocus lens and a zoom lens for use in a retrofocus image pickup optical system. The retrofocus lens has a single focal length and includes a first lens unit having a negative refractive power and a second lens unit having a positive refractive power, which are provided in the stated order from the object side to the image side.
According to U.S. Pat. No. 7,426,079, in order to correct a negative lateral chromatic aberration, a high-dispersion resin material with extraordinary partial dispersion is used for a lens unit provided in the rear of an aperture stop in which the height “h” of the paraxial chief ray entering a lens surface (distance from optical axis) is relatively high. However, since the resin material is used for a lens, a power for the lens alone may not be increased. Therefore, aberrations except for chromatic aberrations cannot be sufficiently corrected. Even when the power of the lens is increased, various aberrations including a distortion at a reference wavelength remain because the lens is low in refractive index, and hence an additional lens for correcting the aberrations is necessary.